Introduction of Feedback Amplifiers

INTRODUCTION OF FEEDBACK AMPLIFIERS AND OSCILLATORS

Harold Black of Western Electric Company invented electronic feedback concept in 1928. When he was working on repeaters for telephone lines. Feedback plays a very important role in electronic circuits and the basic parameters such as input impedance, output impedance, current or voltage gain and bandwidth may be altered considerably by the use of feedback for a given amplifier.

The feedback can be either negative (degenerative) or positive (regenerative). Most desirable performance parameters of the basic amplifiers can be improved when negative feedback is used. Negative feedback is used in amplifiers and filters while positive feedback is used in oscillators.

The only disadvantage of negative feedback is the reduction of transfer gain. The basic idea behind the use of negative feedback is to trade-off transfer gain for other desirable properties.

BASIC CONCEPT OF FEEDBACK

A block diagram of an amplifier with feedback is shown in Fig.4.1.

The output quantity is sampled by a suitable sampler, which is of two types, namely voltage sampler, and current sampler, and fed to the feedback network.

The output of feedback network that has a fraction of the output signal is combined with external source signal φ_s through a mixer and fed to the basic amplifier.

Mixer also called as comparator is of two types namely series comparator and shunt comparator.

Types

There are two types of feedback

(i) Positive feedback

(ii) Negative feedback

i. Positive Feedback (Regenerative Feedback)

If the feedback signal φf is in phase with input signal φ_S then the net effect of the feedback will increase the input signal given to the amplifier. ie., φ_i = φ_S + φ_f. Hence, the input voltage applied to the basic amplifier is increased thereby increasing φ_O exponentially.

Gain of the amplifier with positive feedback is

Here |A_f| > |A|. The product of the open loop gain and the feedback factor is called the loop gain ie loop gain = Aβ. If |Aβ|= 1, then A_f = ∞, hence the gain of the amplifier with positive feedback is infinite and the amplifier gives an ac output without ac input signal. Thus the amplifier acts as an oscillator.

The positive feedback increases the instability of an amplifier reduces the bandwidth and increases the distortion and noise.

The property of the positive feedback is utilized in oscillators.

ii. Negative Feedback (Degenerative Feedback)

If the feedback signals, φf is out of phase with the input signal φ_S. Then φ_i = φ_S- φ_f, so the input voltage applied to the basic amplifier is decreased and correspondingly the output is decreased. Hence, the voltage gain is reduced. This type of feedback is known as negative or degenerative feedback.

Gain of the negative feedback amplifier is

|A_f| < |A|. If |Aβ| >> 1 then A_f = 1/β, where β is a feedback ratio. Hence the gain depends less on the operating potentials and the characteristics of the transistor or vacuum tube.

The gain may be made to depend entirely on the feedback network. If the feedback network contains only stable passive elements the gain of the amplifier using negative feedback is also stable.

Advantages of Negative Feedback

i. Helps to increase the bandwidth

ii. Decreased distortion and noise

iii. Modification of input and output resistance as desired.